1. Field of the Invention
The present invention relates to wireless communications devices, and more particularly, to a wireless communications system that transfers signals at a predetermined call bitrate which is not determined through a maximizing process and a related method of use.
2. Description of the Prior Art
Distance interpersonal communication started with written correspondence over physical media, and has now migrated in modern times to verbal and written correspondence over electronic media. The telephone allows two or more people to communicate verbally over phone lines connected throughout the world by the Public Switched Telephone Network (PSTN). Cellular phones enable wireless verbal communication through wireless connections with base towers that are connected to the PSTN.
Typical, subjective metrics of call quality include volume levels, delay, echo, noise, and speech quality. A good call signal would be characterized by having steady and sufficient volume, no delay, no echoes, no noise, and high speech quality. However, most call system solutions face limitations that require tradeoffs among the quality metrics. A system with exceptional speech quality may suffer in delay. Another system with good noise performance may have echoes.
Conventional phones and cellular phones connected to the PSTN provide stable service, with reasonable quality. However, introduction of new technologies such as Voice over IP (VoIP) enables extra dimensions of communication to speech, including integration with a personal computer (PC), global connectivity, and real-time status of call recipients. By integrating telephone services with a PC, a caller can use a contact list from email software, such as Microsoft Outlook, to store phone numbers. Calls can be logged and recorded directly to a local hard drive in the PC. In terms of connectivity, VoIP enables the caller to make and receive calls from anywhere with network access. Finally, VoIP enables real-time recipient status, which is typically associated with chat programs, such as MSN Messenger or AIM. Using a VoIP device, a caller can tell if a recipient is available, away, busy, or offline. VoIP allows for communication strictly over a network, or through the PSTN. The former is free, whereas the latter usually carries an additional charge.
In such a usage scenario, VoIP devices typically connect to the global Internet or an internal network through a computer running a VoIP software, such as Skype. The VoIP device could be, for example, a microphone and speakers, a headset, or a USB handset, the third of which appears most like a conventional phone. All of these solutions are wired, i.e. each solution connects to the computer, and thereby to the global Internet or the internal network, through a wire or cable.
Of course, by taking advantage of wireless networking technology, such as 802.11a/b/g standard transceivers, a Wireless Local Area Network (WLAN) VoIP device can be realized as a wireless handset identifiable to a networking device, such as a router, by a unique MAC address. In this way, the WLAN VoIP device can exhibit advantages of cordless phones and cellular phones. A further advantage of the WLAN VoIP device over cellular phones is an ability to reduce costs through free calling over the global Internet and low per-minute rates on calls over the PSTN.
One disadvantage of the WLAN VoIP device, however, arises due to an automatic bitrate control, or Adaptive Rate Selection, function of the WLAN. Please refer to FIG. 1, which shows a diagram of wireless signal bitrate throughout a room with various obstructions 10. One factor that affects supportable bitrate is signal attenuation. As a distance between the WLAN VoIP device and the networking device increases, a power of the signal decreases. Obstructions between the WLAN VoIP device and the networking device also decrease a power of the signal. Further, reflections of the signal from walls, floors, and obstructions 10 cause signal interference. As attenuation and interference increase, the bitrate must be reduced to ensure good transmission between the WLAN VoIP device and the networking device. If the WLAN VoIP device is stationary, the bitrate may be affected by people walking in front of the signal. Of course, if the WLAN VoIP device is not stationary, but moving, the supportable bitrate throughout the room causes the Adaptive Rate Selection function to activate. In normal implementation, WLAN VoIP devices or networking devices always start at a highest bitrate to send packets. Whenever the signal quality becomes worse, this causes perceptible degradation in the call signal quality. Particularly, delays can occur in the call signal due to taking several trials to reduce the bitrate to a sustainable one.
To further complicate the issue, the 802.11a/b/g transceiver uses different modulation schemes at different bitrates. For example, in an 802.11g transceiver, an OFDM (orthogonal frequency-division multiplexing) modulation scheme is used at bitrates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. However, at 5.5 and 11 Mbps, a CCK (complementary code keying) modulation scheme is employed. Finally, at 1 and 2 Mbps bitrates, a DBPSK/DQPSK (differential binary/quadrature phase shift keying) modulation scheme is used. Thus, as the WLAN VoIP device adjusts for different supportable bitrates, not only are bitrates adjusted, but modulation schemes change, which further degrades the call signal.